1. Field
The present invention is related to current digital-to-analog converters (DACs) with programmable output current for use in communication systems.
2. Related Art
Communication systems typically include a current digital-to-analog converter (DAC). In order to adjust system characteristics to channel variations, a programmable output current is normally required. One approach to implement a programmable output current is to vary a bias current entering the DAC. FIG. 1 illustrates a common DAC bias that outputs a current source bias voltage 114 to one or more DAC unit current cells, such as unit current cell 200 shown in FIG. 2. When a DAC bias current 103 is changed, a total output current can change proportionately.
An important performance parameter of the DAC is its linearity. Linearity is a function of the matching of the current source transistors (such as transistor 220 of FIG. 2) in the unit current cells. The matching depends on the size (e.g., width by length) of the current source transistors, as well as the biasing condition (e.g., Vgs-Vt). When DAC bias current 103 is reduced, the bias condition (e.g., Vgs-Vt) of the current source transistors is reduced. This results in a deteriorated current matching of the unit current cells. Thus, the maximum range in which DAC bias current 103 can be varied is limited. In a typical configuration, DAC bias current 103 cannot be varied more than 20–30 decibels (dB) before current source matching can dominate the overall performance of the DAC. This range is insufficient for many communication systems.
Therefore, what is needed is a current DAC with programmable current control such that a DAC current control range is extended.